A number of methods are known for reducing bacterial activity in liquids. Traditionally, a so-called "Pasteurization" process is employed, which operates by the principles of thermal denaturation of proteins to inactivate bacteria. Thus, the liquid is raised to a particular temperature for a proscribed duration, to effect a statistical reduction in the number of or even elimination of all viable bacteria. In an effort to reduce a duration of the process, high temperatures may be employed, which raise the temperature of the fluid to, e.g., 150.degree. C. for 2-4 seconds under pressure, followed by a flashing (rapid boiling) to lower the temperature, thus limiting the duration of the treatment. Such systems thus require a very high temperature, and may alter a taste of a potable liquid or food product, such as is the case with milk. Depending on how the heat is applied, precipitation of proteins in the product or other physical changes may occur. In addition, the presence of oxygen during treatment may cause accelerated oxidation.
The most widely used Pasteurized technique involves subjecting food products to heat treatment as high as 65-75.degree. C. and exposing same to this temperature for a period of time of 30 minutes. This is the so-called long-term heat treatment. The second technique involves subjecting food products to heat treatment at a temperature of 70-75.degree. C. and exposing same to this temperature for a period of time of 2-4. minutes. The third technique involves subject food products to short term heat treatment at a temperature of 95.degree. C. and exposing same to this temperature for 30 seconds. The fourth technique includes ultra high temperature heat treatment. It involves subjecting food products to a temperature of 110-140.degree. C. and exposing same to this temperature for a period of time of 2-3 seconds. These treatment are thus based on a thermostability time-temperature relationship of microorganisms. Thermostable life-time is defined as a life-time of microorganisms at a given temperature. The higher the temperature, the shorter the thermostable period. An effective Pasteurization treatment thus subjects food products to heat treatment at a certain temperature for a period of time which is longer than the thermostable period.
These prior art techniques are generally directed toward the thermal denaturation of essential cell elements, they effectively cook the food product, including any biological organisms therewithin. Thus, in addition to altering the taste of the food product, they also affect its composition, for example vitamin concentrations, and structure, for example coagulating proteins or producing sediments. These sediments also necessitate regular cleaning of the system, especially any higher temperature portions, such as heat exchange surfaces.
Some of these drawbacks can be avoided by using the direct heat treatment, which heats the product by way of direct contact of the product subjected to Pasteurization with the heating medium, for instance, steam, rather than through a heat transferring surface of heat exchange equipment. This method eliminates release of the milk "stone" in the heating zone and lessens its appearance on other surfaces of the equipment. These known methods transfer the product into the Pasteurizer, and inject steam made from potable water to a desired temperature, for a desired period. The product is cooled and excess water from condensed steam eliminated. This technique allows a relatively quick heat treatment of the product, and has been found of particular use in ultra high temperature heat treatments. The technique avoids exposure to temperatures higher than a desired final temperature, and thus may limit sedimentation, which may appear, for example, as milk "stone" in a Pasteurization process. Where direct steam contact is used, it dilutes the medium, for example up to 30% of the product mass, with an ultra-high temperature Pasteurization technique, which subsequently is often removed.
These known methods of Pasteurization strive to maintain laminar flow of milk during the process, and thus do not atomize the milk. As a result, these systems fail to raise the temperature of the bulk of the milk at a rapid rate, and rather gradually raise the bulk temperature to the Pasteurization temperature, at which the milk is maintained for the desired period. Of course, a small surface layer may experience rapid temperature rises.
Zhang, et al., "Engineering Aspects of Pulsed Electric Field Pasteurization", Elsevier Publishing Co. (1994) 0260-8774(94)00030-1, pp. 261-281, incorporated herein by reference, relates to Pulsed Electric Field Pasteurization, a non-thermal Pasteurization method. This method (as well as other biological treatment methods) may be combined with other methods, to enhance efficacy of the composite process, while avoiding the limitations of an excess exposure to any one process.